1. Field of the Invention
The present invention relates to ceramic compositions and, more particularly, to composite ceramic compositions comprised of cordierite aluminum magnesium titanate.
2. Technical Background
Refractory materials with low thermal expansion, and consequently high thermal shock resistance, are used in applications such as catalytic converter substrates and diesel particulate filters where high thermal gradients exist during use. One of the best materials for these applications is cordierite due to its low thermal expansion, high melting point, and low cost. In the diesel particulate filter area, it has been recognized that higher heat capacity is desirable for improving durability of filters during regeneration (Hickman SAE). A material with a high volumetric heat capacity is desirable in order to lower the volume of material necessary to absorb a given amount of heat. Less material, volume is desirable because this can reduce pressure drop in the exhaust stream and increase the open volume for ash storage. However, low thermal expansion is still required. Aluminum titanate is one of the few materials that can be made with low thermal expansion and also has higher volumetric heat capacity than cordierite.
Aluminum titanate (AT) and composites containing large fractions of aluminum titanate have several disadvantages. First, pure aluminum titanate is metastable below about 1200° C. Second, the thermal expansion of AT is only low when the grain size is large and microcracks form during cooling in the kiln. These large grains and microcracks tend to make the material mechanically weak. Third, as a consequence of the microcracks, the thermal expansion curve can have very large hysteresis, leading to very high values of instantaneous thermal expansion, especially on cooling. Fourth, the firing temperature of AT-based composites is typically high, usually above 1400° C. Finally, AT has been shown to exhibit very high thermal cycling growth which can be exaggerated by the presence of alkali elements.
To slow down the decomposition rate, additives such as mullite, MgTi2O5, and Fe2TiO5 can be added to the aluminum titanate. MgTi2O5 tends to slow the decomposition rate in reducing conditions and only slows the rate in oxidizing conditions at high levels (>10%). Fe2TiO5 tends to slow the decomposition rate in oxidizing conditions and increase the decomposition rate in reducing conditions (U.S. Pat. No. 5,153,153, 1992).
Second phases such as mullite have been added to AT to increase the strength of the composite body, because microcracking generally does not occur between mullite crystals. Mullite also is a good choice because it also has a fairly high volumetric heat capacity. Other second phases have also been used in AT composites, including alkali and alkaline earth feldspars. However, mullite and alkali feldspars have a higher than optimum thermal expansion.
In an effort to provide a composite AT ceramic body having improved strength while maintaining a low CTE, cordierite would be a better choice than mullite as a second phase because cordierite has a lower coefficient of thermal expansion than does mullite. However, cordierite and pure aluminum titanate are not in thermodynamic equilibrium at any temperature. The provision of a cordierite and AT based composite ceramic having low CTE, high strength, and good thermal stability would represent an advancement in the state of the art. The present invention provides such a body.